In wideband wireless communications systems, data signals tend to deteriorate due to a number of transmission factors—such frequency selective fading due to multi-path transmissions, for example. An OFDM system has been proposed to overcome the problem of frequency selective fading by dividing the total bandwidth into a plurality of subcarriers such that the bandwidth on each subcarrier can carry the data modulation symbols but is sufficiently narrow to experience relatively flat fading.
The OFDMA system uses an OFDM modulation technique to multiplex the traffic data of a plurality of mobile stations in both frequency and time. In a cellular network or an ad hoc network, some mobile stations are moving at a fast speed while the other mobile stations are more stationary when they transmit or receive data. Some mobile stations experience severe fading due to multi-path transmission while others have near line-of-sight channels with a base station antenna. Therefore, two assignment methods have been proposed for an OFDMA-based wireless communications system. A localized assignment method assigns the subcarriers that are contiguous in both time and frequency to one mobile station. A distributed assignment method assigns the subcarriers that are not contiguous in both time and frequency.